The present invention relates to a pneumatic hand tool. More particularly this invention concerns a pneumatically powered drill.
A pneumatically powered hand tool is known provided internally with a pneumatically powered motor or turbine which is connected through a main control valve to a compressed gas inlet on the housing of the tool. This motor carries a tool holder which may be a chuck for drill bits, a mandrel holder for a grinding wheel, a screwdriver bit, or any of a multiplicity of tools. When the main control valve is opened the turbine is pressurized with compressed air and the tool holder is rotated and/or reciprocated.
In order to reduce the wear and initial heating-up in such tools it is known to provide a governor in the housing connected to the motor and operating a valve which reduces or completely cuts off the air supply to the motor when a predetermined speed is exceeded. Such an arrangement prevents the motor from running at extremely high speeds when unloaded, so that wear of the various bearings and waste of compressed gas is minimized.
The principal disadvantage of such a governor-controlled drill is that it is impossible to use it at very high speeds, for instance, when drilling with a small-diameter bit. Thus the provision of such a governor, although it considerably increases the service life of the tool, nevertheless makes the tool unsuitable for some relatively common operations.